A mask is a transparent, e.g. glass or quartz, plate covered with patterns used in making integrated circuits. Each pattern consists of opaque and transparent areas that define the size and shape of all circuit and device elements. The mask is used to expose selected areas of resist, which defines areas to be etched. The term reticle includes mask, especially a photomask, used in wafer fabrication.
A photoresist is a light-sensitive liquid that is spread as a uniform thin film on a wafer or substrate. After baking to solidify the liquid, exposure of specific patterns is performed using a photomask. Material remaining after development shields regions of the wafer from subsequent etch or implant operations.
Lithography is a process of transferring a pattern or image from one medium to another, as from a mask to a wafer. Photolithography is a lithographic technique involving light as the pattern transfer medium.
During the wafer fabrication process of solid state devices, such as a semiconductor, photomasks and reticles are used to form patterns for each layer of the devices on the wafer. Even an extremely small defect created during this process could greatly affect the production yield, circuits reliability or circuits functionality. For this reason, wafers are commonly inspected during each step of the fabrication process.
Some such defects are the result of development process. When resist is exposed, a chemical reaction occurs which results in various chemical by-products. Some of these by-products, residue, are not readily soluble in the developing solution and adhere to the wafer surface. Thus, they can be difficult to remove and they can create defects. Such by-products tend to adhere area of the wafer with relatively less patterns or no patterns.
Because of the possible presence of other types of defects, current in-line photo-track monitor processes result in a low signal to noise ratio for residue defects.
Currently monitor wafers are used to detect defects. These monitor wafers have known repetitious patterns at predetermined locations. These patterns are created using a mask/reticle. This method fails to detect certain types of defects, especially defects relating to the coat and develop portion of the cell. Of particular note, is the failure of this detection method to adequately detect post-develop residue defects. Such residue defects are caused by resist by-products that are not removed during the develop process. Such residue tends to adhere to the wafer surface predominantly in areas with the least amount of patterns, which can be on the substrate or an area of undeveloped resist. Since the repetitious patterns currently used for testing for defects contain patterns, the process is not sensitive to detecting defects such as post-develop residue defects. Also, analyzing a specific pattern for integrity takes a relatively longer amount of time.
A recognized problem, therefore, is detecting defects in the wafer development process, especially post-develop residue defects.
Another recognized problem is the length of time the defect detection process take when an analysis of a pattern must also be performed.